Fluid pressure cylinder and rolling mill

ABSTRACT

Hydraulic cylinders are provided in a housing so as to be opposed to roll chocks. Each of the hydraulic cylinders is constituted by movably fitting a thin plunger via a liner into a cylinder fixed to the housing, movably fitting a concave portion of the thin plunger over a center guide on the cylinder to form guide means, and connecting a liner for thrusting each of the roll chocks to a front end portion of the thin plunger.

[0001] The entire disclosure of Japanese Patent Application No. 2001-234661 filed on Aug. 2, 2001 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a fluid pressure cylinder to be actuated by the supply of an oil pressure or the like, and a rolling mill having the fluid pressure cylinder and adapted to roll a strip material or the like, which passes between upper and lower rolling rolls, to a predetermined thickness.

[0004] 2. Description of Related Art

[0005] In a rolling mill, for example, upper and lower work rolls are rotatably supported inside a housing via work roll chocks, and the upper and lower work rolls are opposed to each other. Upper and lower backup rolls are also rotatably supported inside the housing via backup roll chocks, and the upper and lower backup rolls are opposed to the upper and lower work rolls. A screw down device for imposing a rolling load on the upper work roll via the upper backup roll is provided in an upper portion of the housing.

[0006] Thus, a strip is fed from an entry side of the housing, and passed between the lower work roll and the upper work roll given a predetermined load by the screw down device via the backup roll, whereby the strip is rolled to a predetermined thickness. The rolled strip is delivered from a delivery side of the housing and supplied to a subsequent step.

[0007] In the foregoing rolling mill, hysteresis during vertical control of the work rolls and backup rolls in the housing needs to be minimized in a rolling condition under a screw down load to control the thickness of a rolled plate highly accurately. For this purpose, gaps are formed between the work roll chocks and backup roll chocks and the housing. Thus, even though deformation in an inward narrowing amount is caused to the housing under the screw down load during rolling, gaps are present between the roll chocks and the housing, so that the horizontal dynamic stiffness of the rolling mill may be low. If rolling is performed with a high rolling force and a high percentage reduction in the thickness of the strip while the horizontal dynamic stiffness of the rolling mill is low, great vibrations probably attributed to, for example, friction between the strip being rolled and the work rolls (hereinafter referred to as mill vibrations) occur in the housing or the work rolls, thereby impeding high efficiency rolling. Since the rolls are not stably supported, moreover, rolling becomes unstable, particularly when the strip is engaged between the rolls.

[0008] The applicant of the present application filed Japanese Patent Application No. 2000-187163 (Japanese Unexamined Patent Publication No. 2001-113308) as a solution to the above-described problems. The invention of this application has upper and lower work rolls as a pair and upper and lower backup rolls as a pair rotatably supported in a housing via roll chocks; a screw down device provided in an upper portion of the housing for imposing a predetermined pressure on the upper work roll; and hydraulic cylinder mechanisms provided on an entry side and a delivery side of the housing, the hydraulic cylinder mechanisms being capable of thrusting the roll chocks in a horizontal direction. According to this configuration, the hydraulic cylinder mechanisms are actuated during rolling to eliminate the gaps between the roll chocks and the housing, thereby improving the horizontal dynamic stiffness. As a result, mill vibrations are suppressed, permitting high efficiency rolling.

[0009] The hydraulic cylinder mechanisms for thrusting the roll chocks in the horizontal direction require a huge thrusting force. Further, the hydraulic cylinder mechanisms undergo fluctuating loads, and their working strokes are small. Thus, when the hydraulic cylinder mechanisms are operated, their plungers do not properly move, but tend to stick, in the cylinders. Because of this faulty operation of the hydraulic cylinder mechanisms, mill vibrations cannot be suppressed reliably.

SUMMARY OF THE INVENTION

[0010] The present invention has been accomplished to solve the above problems. Its object is to provide a fluid pressure cylinder improved in operability, and a rolling mill which uses this fluid pressure cylinder to prevent mill vibrations and enable high efficiency rolling, and which, because of the fluid pressure cylinder, stably supports rolls to achieve a stable rolling state.

[0011] As an aspect of the present invention, there is provided a fluid pressure cylinder comprising:

[0012] a cylinder of a cylindrical shape;

[0013] a thin plunger fitted along an inner wall surface of the cylinder so as to be movable in an axial direction;

[0014] an output portion provided at one end portion of the thin plunger; and

[0015] guide means provided between the cylinder and other end portion of the thin plunger by having a concave portion and a convex portion fitted to each other, and adapted to guide movement of the thin plunger relative to the cylinder.

[0016] According to this aspect, the thin plunger is supported by the cylinder so as to be movable by the guide means fitted to the thin plunger because of a convex-concave structure. Thus, the thin plunger is excellent in straight line stability, and properly working without sticking, so that its operability can be improved.

[0017] According to another aspect of the present invention, there is provided a rolling mill comprising:

[0018] a housing;

[0019] upper and lower rolling rolls rotatably supported by the housing via roll chocks;

[0020] screw down means provided in an upper portion of the housing and adapted to apply a predetermined pressure to the rolling roll; and

[0021] hydraulic cylinders for thrusting the roll chocks along a transport direction of a rollable material to press the roll chocks against the housing, and further including

[0022] guide means each provided between a cylinder constituting each of the hydraulic cylinders and a shaft end portion of a thin plunger by having a concave portion and a convex portion fitted to each other, and adapted to guide movement of the thin plunger.

[0023] According to this aspect, the thin plunger is movably supported in the cylinder by the guide means fitted to the thin plunger because of a convex-concave configuration. Thus, the thin plunger is excellent in straight line stability. Even if an axial load or a fluctuating load is imposed by the roll chock during rolling, the thin plunger does not stick, but properly works, and prevents mill vibrations occurring in the housing or rolling rolls. Consequently, the passage of the strip is improved to achieve high efficiency rolling. Since the rolls are stably supported, moreover, a stable rolling state can be realized.

[0024] In the rolling mill, the thin plunger may be movably supported in the housing, and a liner capable of contacting and thrusting the roll chock may be connected to a front end portion of the thin plunger. Since the roll chocks can be directly thrust by the liner, the structure of the rolling mill can be simplified.

[0025] The rolling mill may further include greasing means for greasing a surface of the liner. Thus, abrasion between the liner and the roll chock is diminished, and the operability of the rolling mill can be improved.

[0026] In the rolling mill, the thin plunger may be provided in a liner portion contacting the roll chock, may be movably supported by the liner, and may have a front end portion capable of thrusting the roll chock. Thus, the thin plunger can be easily incorporated by replacing the liner, so that the general-purpose properties of the rolling mill can be improved.

[0027] The rolling mill may further include greasing means for greasing a front end surface of the thin plunger. Thus, abrasion between the liner and the roll chock is diminished, and the operability of the rolling mill can be improved.

[0028] In the rolling mill, the guide means may include a center guide of a cylindrical shape provided at a center of the cylinder, and a concave portion provided in the thin plunger and fitted over the center guide. Thus, a simple structure enables the thin plunger to work properly without sticking, so that structural simplification is achieved.

[0029] In the rolling mill, the thin plunger may be movably fitted on an inner peripheral surface of the cylinder via a liner, a ring-shaped cover may be fixed to the cylinder in correspondence with an outer peripheral portion of the thin plunger, an oil pressure supply chamber may be provided between the cylinder and the thin plunger, and a pullback mechanism may be provided between the cover and the thin plunger. Thus, the return action of the thin plunger is performed by the pullback mechanism, thus obviating the need to machine an oil feed hole, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

[0031]FIG. 1 is a sectional view of a hydraulic cylinder mounted on a rolling mill according to a first embodiment of the present invention;

[0032]FIG. 2 is a front view of the hydraulic cylinder;

[0033]FIG. 3 is a schematic view of the rolling mill according to the present embodiment;

[0034]FIG. 4 is a sectional view of a hydraulic cylinder mounted on a rolling mill according to a second embodiment of the present invention; and

[0035]FIG. 5 is a sectional view of a hydraulic cylinder mounted on a rolling mill according to a third embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which in no way limit the invention.

First Embodiment

[0037] In a rolling mill according to the first embodiment, as shown in FIG. 3, upper and lower work roll chocks 12 and 13 as a pair are supported in a housing 11. Shaft portions of upper and lower work rolls 14 and 15 as a pair are rotatably supported by the upper and lower work roll chocks 12 and 13, respectively, and the upper work roll 14 and the lower work roll 15 are opposed to each other. Upper and lower backup roll chocks 16 and 17 as a pair are supported above and below the upper and lower work roll chocks 12 and 13. Shaft portions of upper and lower backup rolls 18 and 19 as a pair are rotatably supported by the upper and lower backup roll chocks 16 and 17, respectively. The upper backup roll 18 and the upper work roll 14 are opposed to each other, while the lower backup roll 19 and the lower work roll 15 are opposed to each other. A screw down device 20 for imposing a rolling load on the upper work roll 14 via the upper backup roll 18 is provided in an upper portion of the housing 11.

[0038] Hydraulic cylinders (fluid pressure cylinders) 21 and 22 are mounted on an entry side and a delivery side of the housing 11 so as to be opposed to the upper work roll chock 12. The hydraulic cylinders 21 and 22 can be pressed against the housing 11 by thrusting the upper work roll chock 12 from upstream and downstream sides along a transport direction. Hydraulic cylinders (fluid pressure cylinders) 23 and 24 are mounted on the entry side and the delivery side of the housing 11 so as to be opposed to the lower work roll chock 13. The hydraulic cylinders 23 and 24 can be pressed against the housing 11 by thrusting the lower work roll chock 13 from upstream and downstream sides along the transport direction. Hydraulic cylinders (fluid pressure cylinders) 25 and 26 are mounted on the entry side and the delivery side of the housing 11 so as to be opposed to the upper backup roll chock 16. The hydraulic cylinders 25 and 26 can be pressed against the housing 11 by thrusting the upper backup roll chock 16 from upstream and downstream sides along the transport direction. Hydraulic cylinders (fluid pressure cylinders) 27 and 28 are mounted on the entry side and the delivery side of the housing 11 so as to be opposed to the lower backup roll chock 17. The hydraulic cylinders 27 and 28 can be pressed against the housing 11 by thrusting the lower backup roll chock 17 from upstream and downstream sides along the transport direction.

[0039] In the rolling mill of the present embodiment, the hydraulic cylinders 21 to 28 are mounted within the housing 11, and have practically the same configuration. Therefore, only the hydraulic cylinder 21 will be described in detail.

[0040] In the hydraulic cylinder 21, as shown in FIGS. 1 and 2, the housing 11 has a mounting concave portion 31 formed at a predetermined position opposed to the upper work roll chock 12. A cylinder 32 of a cylindrical shape is fitted into the mounting concave portion 31, and is fixed by bolts (not shown). A ring-shaped liner 33 is fitted on the inner peripheral surface of the cylinder 32 via an 0 ring 34. For increased ease of assembly, the liner 33 takes a tapered shape by having an inclined surface 35 formed in its front end outer peripheral portion along the circumferential direction. In a portion of the cylinder 32 opposed to the inclined surface 35, a curved surface portion 36 is formed along the circumferential direction in order to prevent stress concentration. In the liner 33, many oil holes 37 piercing therethrough in the diametrical direction are formed.

[0041] A center guide 38 of a cylindrical shape is fixed at the center of the cylinder 32 by bolts 39. A thin plunger 40 takes a cylindrical shape, and is fitted on the inner wall surface of the liner 33 of the cylinder 32 via a slipper seal 41. A concave portion 42 formed in a base end portion of the thin plunger 40 is fitted over the center guide 38 via an O ring 43 and a slipper seal 44. Thus, the thin plunger 40 is movable in the axial direction. In this case, guide means of the present invention is composed of the center guide 38 and the concave portion 42. A ring-shaped cover 45 is disposed on an outer peripheral portion of the thin plunger 40, and fixed to the cylinder 32 by bolts 46 to fix the liner 33 and define the amount of movement of the thin plunger 40. An oil pressure supply chamber 47 is provided between the base end surface of the thin plunger 40 and the cylinder 32 by a stepped portion. An oil feed passage 48 communicating with the oil pressure supply chamber 47 is formed. A helical compression spring 49, as a pullback mechanism, is interposed between the thin plunger 40 and the cover 45.

[0042] Thus, the thin plunger 40 is normally located at a position of contact with the cylinder 32 by the urging force of the helical compression spring 49 (i.e., the position indicated by solid lines in FIG. 1). When an oil is fed from an oil pressure source (not shown) to the oil pressure supply chamber 47 via the oil feed passage 48, the thin plunger 40 can move rightward in FIG. 1 over a predetermined stroke S in opposition to the urging force of the helical compression spring 49. When oil supply to the oil pressure supply chamber 47 is stopped, the thin plunger 40 moves leftward in FIG. 1 by the stroke S under the urging force of the helical compression spring 49, and the oil is discharged from the oil pressure supply chamber 47.

[0043] The inner wall surface of the liner 33 and the outer wall surface of the thin plunger 40 which are slidable over each other, the outer wall surface of the center guide 38 and the inner wall surface of the concave portion 42 which are slidable over each other, and the outer wall surface of the replaceable liner 33 and the inner wall surface of the cylinder 32 which are slidable over each other are surface treated in consideration of durability, abrasion resistance and impact resistance. When the inner wall surface of the liner 33 is damaged after long-term use, the liner 33 can be replaced.

[0044] The thin plunger 40 has a front end portion to which a flange portion 51 of a support shaft 50 is attached by a support bracket 53 via a compression spring 52. A liner 54, as an output portion, is connected to a front end portion of the support shaft 50 by a connecting member 55, and a cover 56 is attached to the liner 54. In this case, the liner 54 has a curved back surface consistent with the front end surface of the thin plunger 40, and is pivotable in an amount corresponding to the gap between the liner 54 and the support shaft 50.

[0045] Since the liner 54 is designed to thrust the upper work roll chock 12 (see FIG. 3), a ring-shaped greasing groove 57 is formed in the surface of the liner 54 for increased slidability. The greasing groove 57 can be greased from a greasing source (not shown) through greasing passages 58, 59, 60, 61 formed in the cylinder 32, center guide 38, thin plunger 40 and liner 54.

[0046] In the thus constituted rolling mill of the present embodiment, as shown in FIG. 3, a strip is fed from the entry side of the housing 11, and passed between the lower work roll 15 and the upper work roll 14 given a predetermined load by the screw down device 20 via the backup roll 18. As a result, the strip is rolled to a predetermined thickness, delivered from the delivery side, and fed to a subsequent step. During such rolling of the strip, an inward narrowing deformation amount occurs in the housing 11 in response to a screw down load. However, a thrusting force is exerted on the housing 11 by actuating the hydraulic cylinders 21 to 28, whereby the deformation amount of the housing 11 is decreased.

[0047] That is, as shown in FIG. 1, the oil is supplied from the oil pressure source to the oil pressure supply chamber 47 via the oil feed passage 48. As a result, the thin plunger 40 moves over the predetermined stroke S under a predetermined oil feed pressure in opposition to the urging force of the helical compression spring 49, thereby thrusting the upper work roll chock 12 via the liner 54. At this time, the thin plunger 40 moves, with its outer peripheral surface being supported by the liner 33, and with the concave portion 42 at its based end portion being supported by the center guide 38. Thus, its straight line stability is fully ensured, and the thin plunger 40 can move properly without sticking. When the thin plunger 40 thrusts the upper work roll chock 12 via the liner 54, the liner 54 pivots following the upper work roll chock 12, or a reaction force against rolling acts on the thin plunger 40 via the upper work roll chock 12. However, the thin plunger 40 has the concave portion 42 supported by the center guide 38. Thus, the thin plunger 40 can reliably accept this reaction force and properly move.

[0048] Hence, even if the roll chocks 12, 13, 16, 17 are displaced, no gaps occur between the roll chocks and the housing 11. As a result, the horizontal dynamic stiffness of the rolling mill is kept high. Even when rolling is performed in this state with a high rolling force and a high percentage reduction in the plate thickness, great mill vibrations probably attributed to, for example, friction between the strip and the work rolls 14, 15 do not occur in the housing 11 or the work rolls 14, 15, thus permitting high efficiency rolling. Moreover, the rolls are stably supported to realize a stable rolling state.

[0049] In the present embodiment, as described above, the hydraulic cylinders 21 to 28 are mounted on the entry side and delivery side of the housing 11 so as to be opposed to the roll chocks 12, 13, 16, 17. These hydraulic cylinders 21 to 28 are constituted in the following manner: The thin plunger 40 is movably fitted into the cylinder 32 fixed to the housing 11 via the liner 33. The concave portion 42 of the thin plunger 40 is movably fitted over the center guide 38 of the cylinder 32 to serve as guide means. The liner 54 for thrusting each of the roll chocks 12, 13, 16, 17 is connected to the front end portion of the thin plunger 40.

[0050] The thin plunger 40 has the outer peripheral portion and the central portion movably supported by the cylinder 32 in the above manner. Thus, the thin plunger 40 is excellent in straight line stability. Even if an axial load or a fluctuating load is imposed by the roll chocks 12, 13, 16, 17 during rolling, the thin plunger 40 does not stick, but properly works, and prevents mill vibrations occurring in the housing 11 or work rolls 14, 15. Consequently, the passage of the strip is improved to achieve high efficiency rolling, and a stable rolling state can be realized.

[0051]FIGS. 4 and 5 show sections of hydraulic cylinders mounted on rolling mills according to other embodiments of the present invention. Members having the same functions as described in the aforementioned embodiment are assigned the same numerals, and duplicate explanations are omitted.

Second Embodiment

[0052] In a rolling mill according to a second embodiment, as shown in FIG. 4, a thin plunger 40 is fitted on the inner wall surface of a liner 33 of a cylinder 32 via a slipper seal 41. A concave portion 42 of the thin plunger 40 is fitted over a center guide 38 via an O ring 43 and a slipper seal 44. Thus, the thin plunger 40 is movable in the axial direction. An oil pressure supply chamber 47 is provided between the base end surface of the thin plunger 40 and the cylinder 32, and an oil feed passage 48 communicates with the oil pressure supply chamber 47. An oil pressure supply chamber 65, as a pullback mechanism, is provided among the thin plunger 40, a cover 45 and the liner 33, and an oil feed passage 66 communicates with the oil pressure supply chamber 65. The oil feed passages 48, 66 are connected to an oil pressure source via selector valves (not shown).

[0053] The selector valves are operated to supply an oil from the oil pressure source to the oil pressure supply chamber 47 via the oil feed passage 48, and discharge the oil from the oil pressure supply chamber 65 via the oil feed passage 66. As a result, the thin plunger 40 is moved rightward in FIG. 4, whereby a liner 54 can thrust the roll chock. On the other hand, the selector valves are switched to supply the oil from the oil pressure source to the oil pressure supply chamber 65 via the oil feed passage 66, and discharge the oil from the oil pressure supply chamber 47 via the oil feed passage 48. As a result, the thin plunger 40 is moved leftward in FIG. 4, whereby the liner 54 can be stopped from thrusting the roll chock.

[0054] In the present embodiment, as described above, the movement of the plunger 40 is performed only by the supply and discharge of the oil pressure. By so doing, the action of the plunger 40, namely, its action of thrusting the roll chock, can be carried out reliably.

[0055] In a rolling mill according to a third embodiment, as shown in FIG. 5, a liner 72 is supported on a curved surface 71 of a housing 11 so as to be opposed to a roll chock 12 and be pivotable by a pivot mechanism (not shown). A hydraulic cylinder 73 is mounted on the liner 72.

[0056] That is, the cylinder 73 is formed at a predetermined position of the liner 72, and a concave portion 74 of a cylindrical shape is formed at the center of the cylinder 73. A thin plunger 75 is disc-shaped, and is fitted on the inner wall surface of the cylinder 73 via a slipper seal 76. A center guide 77 formed in a base end portion of the thin plunger 75 is fitted into the concave portion 74 via a slipper seal 78. Thus, the thin plunger 75 is movable in the axial direction. In this case, guide means of the present invention is composed of the concave portion 74 and the center guide 77. A cover 79 is disposed on an outer peripheral portion of the thin plunger 75, and fixed to the liner 72 by a bolt 80 to define the stroke S of the thin plunger 75. An oil pressure supply chamber 81 is provided between the base end surface of the thin plunger 75 and the liner 72, and an oil feed passage 82 communicates with the oil pressure supply chamber 81. A helical compression spring 83, as a pullback mechanism, is interposed between the thin plunger 75 and the cover 79.

[0057] Moreover, the thin plunger 75 has a front end portion to which a front end liner 84, as an output portion, is fixed by a bolt 85. The thin plunger 75 can thrust the roll chock via the front end liner 84. A greasing groove 86 of a cross shape is formed in the surface of the front end liner 84 for increased slidability. The greasing groove 86 can be greased from a greasing source (not shown) through greasing passages 87, 88 formed in the thin plunger 75, front end liner 84, etc.

[0058] Thus, when an oil is supplied from the oil pressure source to the oil pressure supply chamber 81 via the oil feed passage 82, the thin plunger 75 moves over a predetermined stroke S under a predetermined oil feed pressure in opposition to the urging force of the helical compression spring 83, thereby thrusting the roll chock via the front end liner 84. At this time, the thin plunger 75 moves, with its outer peripheral surface being supported by the cylinder 73, and with the center guide 77 being supported by the concave portion 74. Thus, its straight line stability is fully ensured, and the thin plunger 75 can move properly without sticking.

[0059] In the present embodiment, as described above, the liner 72 is pivotably supported by the housing 11, the cylinder 73 is formed in the liner 72, and the thin plunger 75is movably fitted into the cylinder 73. The center guide 77 of the thin plunger 75 is movably fitted into the concave portion 74 of the cylinder 73 to serve as guide means. The front end liner 84 for thrusting the roll chock is fixed to the front end portion of the thin plunger 75. Thus, even the existing equipment can easily incorporate the thin plunger 75 by replacing the liner 72, and can be improved in general versatility.

[0060] In the aforementioned first embodiment, the center guide 38 is formed as a separate member, and fixed to the cylinder 32 by the bolts 39. In the third embodiment, the center guide 77 is formed integrally with the thin plunger 75. The center guide may be a separate member or an integral member, which may be selected, as desired, according to the production cost or ease of assembly. The position of formation of the center guide and the concave portion may be in the thin plunger or in the cylinder. The guide means is the center guide and the concave portion. However, instead of the center guide, a plurality of convex portions may be provided at positions other than the central position.

[0061] Besides, the rolling mill of the present invention is preferably used not only for rolling mills of ordinary shapes, but also for cross rolling mills and shift rolling mills. Furthermore, the fluid pressure cylinder of the present invention is applied as the hydraulic cylinders 21 to 28 for thrusting the roll chocks in various rolling mills. However, the fluid pressure cylinder may be provided at any place where its working force is great and its working stroke is short.

[0062] While the present invention has been described in the foregoing fashion, it is to be understood that the invention is not limited thereby, but may be varied in many other ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims. 

What is claimed is:
 1. A fluid pressure cylinder comprising: a cylinder of a cylindrical shape; a thin plunger fitted along an inner wall surface of the cylinder so as to be movable in an axial direction; an output portion provided at one end portion of the thin plunger; and guide means provided between the cylinder and other end portion of the thin plunger by having a concave portion and a convex portion fitted to each other, and adapted to guide movement of the thin plunger relative to the cylinder.
 2. A rolling mill comprising: a housing; upper and lower rolling rolls rotatably supported by the housing via roll chocks; screw down means provided in an upper portion of the housing and adapted to apply a predetermined pressure to the rolling roll; and hydraulic cylinders for thrusting the roll chocks along a transport direction of a rollable material to press the roll chocks against the housing, and further including guide means each provided between a cylinder constituting each of the hydraulic cylinders and a shaft end portion of a thin plunger by having a concave portion and a convex portion fitted to each other, and adapted to guide movement of the thin plunger.
 3. The rolling mill of claim 2, wherein the thin plunger is movably supported in the housing, and a liner capable of contacting and thrusting the roll chock is connected to a front end portion of the thin plunger.
 4. The rolling mill of claim 3, further including greasing means for greasing a surface of the liner.
 5. The rolling mill of claim 2, wherein the thin plunger is provided in a liner portion contacting the roll chock, is movably supported by the liner, and has a front end portion capable of thrusting the roll chock.
 6. The rolling mill of claim 5, further including greasing means for greasing a front end surface of the thin plunger.
 7. The rolling mill of claim 2, wherein the guide means includes a center guide of a cylindrical shape provided at a center of the cylinder, and a concave portion provided in the thin plunger and fitted over the center guide.
 8. The rolling mill of claim 2, wherein the thin plunger is movably fitted on an inner peripheral surface of the cylinder via a liner, a ring-shaped cover is fixed to the cylinder in correspondence with an outer peripheral portion of the thin plunger, an oil pressure supply chamber is provided between the cylinder and the thin plunger, and a pullback mechanism is provided between the cover and the thin plunger. 